Polymerase zeta dependency of increased adaptive mutation frequencies in nucleotide excision repair-deficient yeast strains.

Reversions of an auxotrophy-causing frameshift allele during prolonged starvation of yeast cells were used as a means to elucidate the mechanisms concerned with the generation of spontaneous adaptive mutations in cell cycle-arrested cells. Whereas about 50% of these reversions were previously shown to depend on the non-homologous end joining pathway of DNA double-strand break repair, the origin of the residual 50% remains unknown. In search for a mechanism for generation of the latter fraction of reversions we examined the role of the translesion synthesis (TLS) polymerases zeta, eta and Rev1p in cells with wild-type or impaired nucleotide excision repair (NER) capacity. The basal level of adaptive mutations in the repair-proficient wild type was not influenced by disruptions of the genes coding for these three TLS polymerases. Intriguingly, a deficiency in NER by disruption of RAD14, RAD16 or RAD26 resulted in a significantly higher frequency of adaptive mutation, yet this increase was strictly dependent on an intact REV3 gene, coding for the catalytic subunit of polymerase zeta. Furthermore, we observed that intact REV3 was also required for the occurrence of increased frequencies of adaptive mutants in the NER-proficient wild type following UV irradiation. While in proliferating cells the translesion synthesis function of polymerase zeta is connected to DNA replication, our data suggest that in cell cycle-arrested cells this enzyme is able to carry out either TLS or error-prone polymerization along an undamaged template in the course of repair processes. Such a hitherto unappreciated activity of polymerase zeta in non-replicating cells may contribute to the incidence of mutations in evolution, aging and cancer.

Non-homologous end joining as an important mutagenic process in cell cycle-arrested cells.

Resting cells experience mutations without apparent external mutagenic influences. Such DNA replication-independent mutations are suspected to be a consequence of processing of spontaneous DNA lesions. Using experimental systems based on reversions of frameshift alleles in Saccharomyces cerevisiae, we evaluated the impact of defects in DNA double-strand break (DSB) repair on the frequency of replication-independent mutations. The deletion of the genes coding for Ku70 or DNA ligase IV, which are both obligatory constituents of the non-homologous end joining (NHEJ) pathway, each resulted in a 50% reduction of replication-independent mutation frequency in haploid cells. Sequencing indicated that typical NHEJ-dependent reversion events are small deletions within mononucleotide repeats, with a remarkable resemblance to DNA polymerase slippage errors. Experiments with diploid and RAD52- or RAD54-deficient strains confirmed that among DSB repair pathways only NHEJ accounts for a considerable fraction of replication-independent frameshift mutations in haploid and diploid NHEJ non-repressed cells. Thus our results provide evidence that G(0) cells with unrepressed NHEJ capacity pay for a large-scale chromosomal stability with an increased frequency of small-scale mutations, a finding of potential relevance for carcinogenesis.